This invention relates to making cellular resin products having substantially closed cell structure from radiation-crosslinked high density polyethylene.
It is known from U.S. Pat. No. 3,470,119, C. J. Benning et al., to make molded foam articles from high density polyethylene which is crosslinked during the foaming step by action of chemical crosslinking agents, e.g., certain organic peroxides. In such technology, a uniform mixture of polyethylene, chemical crosslinking agent and foaming agent is shaped at a temperature below the decomposition temperature of such agents, which agents decompose at similar temperatures above the melting point of the polyethylene, and the so-shaped mixture is heated to the decomposition temperature of said agents in the substantial absence of atmospheric oxygen, e.g., in an atmosphere of nitrogen gas. Exclusion of oxygen is necessary because oxygen was found to seriously inhibit the crosslinking reaction by the chemical crosslinking agents. However, such technology is disadvantageous because it requires successfully performing the trick of carrying out the chemical crosslinking reaction and the foaming step at precisely the same time. Moreover, when the process is successful, the polyethylene matrix in the resulting foam product is chemically crosslinked. The technology of U.S. Pat. No. 3,470,119 is not applicable to radiation-crosslinked polyethylene because polyethylene which has been pre-crosslinked by radiation cannot be uniformly mixed with blowing agent or shaped by heat without damage to the crosslinked structure.
It is known from U.S. Pat. No. 3,562,367, Shinohara et al., to make foamed sheet from radiation-crosslinked polyethylene. According to this patent, a non-cellular foamable sheet is made by forming a mixture comprising polyethylene and chemical foaming agent such as azodicarbonamide, melt-blending the mixture without decomposing the chemical foaming agent, forming the melt-blended mixture into a sheet, and irradiating the sheet with ionizing radiation to effect partial crosslinking thereof. The resulting non-cellular, foamable, radiation-crosslinked sheet is then caused to foam by floating it on the surface of a heated liquid bath while concurrently heating the upper surface of the sheet by infra-red heaters thereabove. The process gives good results in uniform closed-cell foam sheet when the starting polymer is low-density polyethylene, and the process is alleged to be operable with other kinds of polyethylene. However, when the starting foamable sheet is composed of linear, highly crystalline, high density polyethylene, float-foaming in the manner of U.S. Pat. No. 3,562,367 does not produce a satisfactory foamed sheet product. The foamed product tends to collapse leading to non-uniform foam, irregular cell size and foam density, rough, irregular surface, and distorted shape. Also, the foam lacks the strength and toughness that would be expected from linear, high density polyethylene.
The result just described is not specifically related to heating by floating the sheet on molten salt. A similar unacceptable result is obtained when foamable irradiation-crosslinked sheet composed of linear high density polyethylene and chemical foaming agent is heated in air in the manner described in U.S. Pat. No. 3,711,584, Sagane et al. Although the method of that patent gives uniform foam sheet when the irradiation-crosslinked composition is based on low density polyethylene, such result is not obtained when the starting foamable sheet is based on high density polyethylene.
The art prior to the present invention has not shown means for making cellular resin products of uniform quality with good physical properties from radiation-crosslinked high density polyethylene, especially in the form of sheets, although such products would be desirable.
It is an object of this invention to provide cellular resin products, having substantially closed cell structure, uniform quality and good physical properties, from radiation-crosslinked high density polyethylene.
Another object is to provide such means to make such radiation-crosslinked high density polyethylene foam in the form of sheet having uniform foam structure and smooth, regular surfaces on both sides, free of distortion.
Other objects and advantages of the invention will be brought out in the description that follows.